FIG. 9 of the attached drawings is a schematic perspective view of an electric discharge machining apparatus having a C-shaped configuration in side elevational view. In this conventional example, the electric discharge machining apparatus is a die-sinking electric discharge machining apparatus. The electric discharge machining apparatus 1 comprises a bed 2, and a saddle 3 and a table 4 which are mounted on the bed 2. A processing tank 5 is mounted on the table 4. A head 7 is mounted on a column 6 which extends from the bed 2. A processing electrode 8 is fixedly mounted on the head 7 through an electrode-mounting stool 9. Further, the reference numeral 10 denotes a Y-axis driving motor; 11, an X-axis driving motor; 12, a processing liquid reserved within the processing tank 5; 13, a table stool on which a workpiece (not shown) is fixedly mounted and which is a plate-like element generally.
FIG. 10 is a partially schematic cross-sectional view of the conventional processing tank 5 which is disclosed in, for example, Japanese Patent Laid-Open No. HEI 2-48113. Of the processing tanks 5 which are movable vertically in a telescopic manner, FIG. 10 shows an example in which the processing tank 5 is movable vertically in a three-step or three-stage manner. The reference numerals 14, 15 and 16 denote an outer wall, a bottom wall and an inner wall of a first stage in the processing tank 5, respectively; 17 and 18, an outer wall and an inner wall of a second stage in the processing tank 5, respectively; and 19, 20 and 21, an outer wall, an upper surface and an inner wall of a third stage in the processing tank 5, respectively.
Further, the reference numeral 22 denotes a seal element arranged between the table stool 13 and the bottom surface 15 of the first stage in the processing tank 5; 23, a seal element arranged between the inner wall 16 of the first stage in the processing tank 5 and the inner wall 18 of the second stage in the processing tank 5; and 24, a seal element arranged between the inner wall 18 of the second stage in the processing tank 5 and the inner wall 21 of the third stage in the processing tank 5. The bottom surface 15 of the first stage in the processing tank 5 forms a bottom surface of the entire processing tank 5, while the upper surface 20 of the third stage in the processing tank 5 forms an upper surface of the entire processing tank 5.
FIG. 11 is a schematic side elevational view showing a processing-tank vertically-moving unit of the conventional electric discharge machining apparatus, and shows the processing tank 5 as viewed from the left-hand side in FIG. 10. A support shaft 25 is fixedly mounted on the outer wall 17 of the second stage in the processing tank 5. A pair of arms 26 supported by the support shaft 25 and a pair of guide elements 28 having respective guide grooves in guide pins 27 mounted respectively on forward ends of the arms 26 are mounted on the outer wall 19 of the third stage in the processing tank 5. A ball screw 29 and a pair of nuts 30 are mounted on the outer wall 14 of the first stage in the processing tank 5, and holds or retains the arms 26.
Operation will next be described. As shown in FIG. 9, the saddle 3 moves on the bed 2 in a Y-axis direction by the Y-axis driving motor 10. The table 4 moves on the saddle 3 in an X-axis direction by the X-axis driving motor 11. The electrode-mounting stool 9 moves in a Z-axis direction by a Z-axis driving motor (not shown) within the head 7. Accordingly, the workpiece (not shown) mounted on the table stool 13 can move in the X- and Y-directions, while the processing electrode 8 mounted on the electrode-mounting stool 9 can move in the Z-axis direction. The processing liquid 12 is reserved within the processing tank 5, and electrode discharge processing is executed at a location between the processing electrode 8 and the workpiece through the processing liquid 12.
As shown in FIG. 10, the outer wall 14, the bottom surface 15 and the inner wall 16 of the first stage in the processing tank 5 are fixedly mounted on the table stool 13 through the seal element 22, so that the outer wall 14, the bottom surface 15 and the inner wall 16 do not move vertically. On the other hand, the outer wall 17 and the inner wall 18 of the second stage in the processing tank 5 move vertically by a processing-tank drive unit illustrated in FIG. 11, in integral relation to each other. Further, the outer wall 19, the upper surface 20 and the inner wall 21 of the third stage in the processing tank 5 similarly move vertically. Accordingly, it is possible to optionally vary a vertical height of the entire processing tank 5.
As shown in FIG. 11, a so-called "link mechanism" is formed by the support shaft 25, the arms 26, the guide pins 27, the guide elements 28, the ball screw 29 and the nuts 30, thereby forming the processing-tank vertically-moving unit. The ball screw 29 mounted on the outer wall 14 of the first stage in the processing tank 5, which is not moved vertically, is rotated by a processing-tank vertically-moving motor (not shown), and the nuts 30 move in their respective directions different from each other, whereby the arms 26 held respectively by the nuts 30 are open and closed. As a result, the support shaft 25, that is, the outer wall 17 of the second stage in the processing tank 5 and the outer wall 19 of the third stage in the processing tank 5 move vertically, making it possible to optionally vary the vertical height of the entire processing tank 5.
Furthermore, as shown in FIG. 10, the first stage and the second stage in the processing tank 5 cooperate with each other to form a sealed structure by the seal elements 23 and 24. Accordingly, if the processing liquid 12 is supplied into the processing tank 5, it is possible to reserve the processing liquid 12 in a space defined by the processing tank 5 and the table stool 13. Moreover, generally, an amount of the processing liquid 12 reserved within the processing tank 5, that is, a height of a liquid surface varies depending upon a vertical height of the processing tank 5. It is usual that program operation such as raising of the processing tank 5 to a desirable location, reservation of the processing liquid 12 within the processing tank 5 to execute discharge processing, mounting and demounting of the electrode/workpiece, and the like is executed under such a condition that the processing tank 5 is lowered.
FIG. 12 is a conceptional view of the link mechanism in the processing-tank vertically-moving unit illustrated in FIG. 11. In FIG. 12, the reference character F denotes a load which is vertically supported by the processing-tank vertically-moving unit, and which is equivalent to the tare or dead weight of the processing tank 5. In the normal or usual processing-tank vertically-moving unit, two sets of link mechanisms are arranged in front of and in rear of the processing tank 5, respectively, or on either side of the processing tank 5, respectively. In this case, the load F is half the tare weight of the processing tank 5. The reference character 1 denotes a length of the arm 26; .theta., an angle defined between the arms 26 and the horizontal line; H, a height of the processing tank 5; T, a retaining force in the horizontal direction for supporting the load F by the link mechanism. As shown in FIG. 11, the retaining force T is supported by the processing-tank vertically-moving drive unit which comprises the ball screw 29, the nuts 30 and the processing-tank vertically-moving motor (not shown). The following relationship exists among the above-described characters: EQU H =1.multidot.sin .theta. EQU T =F/(2.multidot.tan .theta.)
FIG. 13 is a graphical representation of the above-described relationship. The ordinate shows the retaining force T, while the abscissa shows the angle .theta. and the height H in combination. In this connection, the load F =50 kgf, while the length 1=300 mm, as a representative example.
Further, FIG. 14 is a view for explanation showing another conventional example of the electric discharge machining apparatus. As shown in FIG. 14, the reference numeral 41 denotes a processing tank for storing or reserving therein a processing liquid 50, which has a side surface and a bottom surface formed of metal sheet; 42, a table supported on a bed 44 by a support table 43; 45, a packing which seals a location between the support table 43 and the bottom surface of the processing tank 41 and which slidingly moves vertically, so that a surface of the support table 7 is processed in grinding; 46, four (4) ball screws which are mounted respectively on an outer periphery of the bottom surface of the processing tank 41; 47, four (4) sprockets which are mounted respectively on lower ends of the ball screws 46; 48, a chain mounted on the sprocket 47; and 49, a geared motor for driving the chain 48.
Operation will next be described. When the processing tank 41 is lowered to a lower limit or a lowermost position, the table 42 is exposed out of the processing liquid 50 which is reserved within the processing tank 41, as shown in the left-hand portion in FIG. 14, so that a condition is brought to one under which the workpiece can be mounted on the table 42. After the workpiece has been mounted on the table 42, the geared motor 49 starts. The chain 48 is mounted on the geared motor 49. By rotation of the sprockets 47 which are mounted on the lower end of the ball screw 46, the chain 48 runs to rotate the four ball screws 46 in synchronism with each other in this case. Since nut sections of the respective rotating ball screws 46 are mounted on the bottom surface of the processing tank 41, rotation of the ball screws 46 raises or moves upwardly the processing tank 41 as shown in the right-hand portion in FIG. 14. Four (4) locations on the bottom surface of the processing tank 41 are maintained at the same height by the four (4) ball screws 46. Also, a clearance between the packing 45, which is mounted on the bottom surface of the processing tank 41, and the support table 43, which has a surface thereof that is polished or ground, is maintained constant.
The first conventional electric discharge machining apparatus arranged as described above has the following problems. That is, the processing-tank vertically-moving unit is large-sized so that the manufacturing cost of the machine is high. Further, operability at the time the electric discharge machining apparatus is used is deteriorated.
The above-discussed problems will be described in detail with reference to FIG. 13. Specifically, according to FIG. 13, the load F which should be supported by the processing-tank vertically-moving unit is 50 kgf, whereas the retaining force T supported by the processing-tank vertically-moving drive unit is smaller than 50 kgf at the angle .theta.&gt;26.6 deg. (height H &gt;134 mm), but is larger than 50 kgf at the angle .theta.&lt;26.6 deg. (height 134 mm). For example, if it is assumed that the angle .theta. under a condition that the processing tank 5 is lowered to the lowest position .theta.=10 deg. (height H.sub.L =52 mm), the retaining force T at this time reaches T.sub.L =142 kgf. This load is approximately thrice the load F which should originally be supported by the processing-tank vertically-moving unit. Thus, the following problems arise. That is, generally, the processing-tank vertically-moving unit, which comprises the ball screw, the nut and the processing-tank vertically-moving motor, is designed on the basis of the maximum retaining force T.sub.L which should be supported. Accordingly, the processing-tank vertically-moving drive unit is large-sized. As a result, the entire processing-tank vertically-moving unit is large-sized, so that the manufacturing cost of the machine is raised.
Further, the following problems arise. That is, as the processing-tank vertically-moving unit is large-sized, the processing tank, in which the processing-tank vertically-moving unit is housed, is also large-sized. If the processing tank is large-sized, accessibility or approachability is reduced with respect to the electrode mounted on the electrode-mounting stool and the workpiece on the table stool. Operability, at the time the electric discharge machining apparatus is used, also is reduced.
Moreover, the second conventional electric discharge machining apparatus constructed as described above has the following problems. That is, in order to seal the location between the processing tank and the support table, it is necessary to grind the surface of the support table, and a relative distance between the processing tank and the support table is required to be maintained constant. Accordingly, the machine becomes very high in price or cost, and the reliability of the machine is lowered.